The present invention relates to a stereo amplifier circuit suitably used with stereo headphones, wherein the amplified stereo signals of the left and right channels appear at three output terminals and the output power is prominently increased under a low rated power supply voltage.
A stereo amplifier circuit for amplifying bichannel stereo signals and supplying the amplified signals to a 3-input speaker apparatus, such as a stereo headphone, is widely used. In a conventional stereo amplifier circuit of this type, when the absolute value of power supply voltage applied to the amplifier circuit is .vertline.Vcc.vertline., the L-channel and R-channel output voltages of the amplified signals are at most .+-.Vcc (peak) with respect to the ground potential (0 V). If large output voltages are required, the power supply voltage must be increased. However, since an amplifier circuit for a stereo headphone is generally embodied in battery-operated portable equipment, the power supply voltage is restricted by the number of batteries used therein and hence, a high output power is hardly obtained according to a conventional battery-operated stereo amplifier circuit provided for three output terminals.
As a typical approach to increase the output voltage of the L- and R-channels with a low power supply voltage, a BTL (balanced transformerless) circuit is conventionally known. According to the BTL circuit, when the absolute value of the power supply voltage is .vertline.Vcc.vertline., the output voltage of nearly 2 Vcc (peak) can be obtained. However, in a stereo amplifier circuit using the BTL circuit arrangement, four output terminals are inevitably used. As a result, a conventional BTL type stereo amplifier circuit cannot be adapted to 3-input stereo headphones.